Bleaching of coral

Since the early 80's, it has been discovered a massive dysfunction of reef organs. It affects not only corals, but also other algae's. Coral loses its colors, hence the common name "Bleaching".

braincoralThe first described mass bleaching began in Bonaire in June 1979 and ended in February 1980. It occurred on the windward coast of Bonaire from 10 to 40 m depths. Bleaching also occurred in several areas in Florida and the Great Barrier Reef. In 1987-1988, the bleaching occurred worldwide and was the most extensive bleaching ever recorded.

Braincoral Recent mass bleaching took always place during the warmest part of the year. The temperature is in most cases characterized as "above normal". Together with experimental evidences that raising the temperature above local normal can induce bleaching, this has lead to consider temperature as the critical factor. Mass bleaching might be the first signal of global warming! It is still unknown if the temperature is the cause of mass bleaching.

High level of irradiance was also suggested as a cause of mass bleaching. In addition to photosynthesis enhancement, light may also warm the upper face. Dozens of reports disproved this suggestion after a research with turbid water.

spongeHigh UV irradiance has been suggested to participate in bleaching. The most difficult fact to reconcile with the UV hypothesis is the occurrence of bleaching at 90 meters. Bleaching also occurred in an underwater cave. An experiment with UV-opaque and UV-transparent plastic plates at 18 meters, yielded similar less bleaching of larger foraminifers underneath both of them after four months, but densities increased only under UV-opaque shields.






The main knowledge on the reef mass bleaching phenomenon may be summarized as follows:

  • It is a global phenomenon, observed in shallow marine tropical areas;
  • It is very recent and sudden (since 1979);
  • It is probably increasing in magnitude and becoming chronic;
  • no coherent spatial patterns emerge (depth, reef zonation, geographic area), except maybe a relationship with groove structures and pass ;
  • It affects almost all, and probably all, and quasi-exclusively animal-algae symbioses which constitute the founder of reef ecosystems;
  • In very general term, it affects preferentially fast-growing, or better said high-photosynthesizing associations;
  • It affects them in mass, with subsequent variable mortality.

After more than a decade of mass bleaching events, it has been not possible to identify clearly its origin, though it is widely believed to be related to the "Global Changes":

  • It is generally associated with warmest temperatures, but not with exceptional ones in a few well-studied cases. It is yet difficult to relate it to global warming;
  • It is frequently associated with calm weather, which might have various consequences (on hydrological patterns, water transparency, air-sea exchanges, and physiology). Changes in the last decade of other weather parameters during bleaching remain to be studied;
  • Light is often involved, probably in relation to photosynthesis;
  • UV can not be directly, and hardly indirectly, responsible for bleaching, as they have not yet increase in tropics;
  • Carbon dioxide build up remains as the last serious hypothesis, but rather because its effects are almost unknown.

Mass bleaching is a threat on all the reef ecosystems, potentially more dangerous than every other disturbance, such as sea level rise or sea level fall, local pollution or future UV increase. Lack of explanation of the phenomenon makes it even more alarming than impact of ozone depletion in polar and temperate regions or acid rain on temperate forests, for which remedies are known ...

In 2012 a report of the International Union for Conservation of Nature (IUCN), showed that around the islands of Dutch Caribbean (Bonaire, Saba and Sint Eustatius) with Aruba, Curacao and Sint Maarten, only 30 % of the coral is still alive.

Special thanks to Erez Wolf for his underwater pictures.


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